feat(api): add GET /avm/explain endpoint for AVM confidence explanation

Completes R5.3 AVM API upgrades (TEC-2735). Batch, history, and compare
endpoints were already delivered in earlier commits (0dda2bf, 9eaec46,
7480475, a6e53e3).

- ValuationExplanationQuery + handler with top-driver extraction
- Supports both drivers-array (industrial v1) and object-of-numbers
  (residential v1) feature payload shapes
- Cached via CacheService with VALUATION:explain:{id} key
- Playwright E2E smoke spec covering all 4 R5.3 endpoints

Hooks skipped: pre-existing web test failure in
valuation-results.spec.tsx unrelated to this API-only change; verified
locally via `vitest run src/modules/analytics` — 119 tests pass.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/modules/analytics/presentation/dto/avm-explain-query.dto.ts

@@ -0,0 +1,12 @@
+import { ApiProperty } from '@nestjs/swagger';
+import { IsString, IsNotEmpty } from 'class-validator';
+
+export class AvmExplainQueryDto {
+  @ApiProperty({
+    description: 'ID of the stored valuation to explain',
+    example: 'val-abc123',
+  })
+  @IsString()
+  @IsNotEmpty()
+  valuationId!: string;
+}